Portable chemical sensors generated from liquid crystals

Exposure to a targeted low molecular-weight compound drives a change in orientation of the liquid crystals.(24K JPG)

The behavior of liquid crystals provides the foundation for highly sensitive, very small and potentially wearable chemical sensors that could give visual warning of a wide range of dangers in real-time.

Nick Abbott and former graduate student Rahul R. Shah, now at 3M Corporation, describe the device in the August 17 issue of Science. The sensors could be used to measure personal exposure to chemicals, including pesticides and chemical warfare agents, in a variety of environments.

Currently, portable devices for detecting toxins in the air weigh tens of pounds and cost thousands of dollars. Abbott and Shah's liquid crystal detectors could be made into small inexpensive badges that change color or display a message when in the presence of a toxin or, for example, the chemicals that appear as meat or fish begins to spoil.

"In flat panel displays, electric fields change the orientation of liquid crystals to produce different optical appearances," says Abbott. "Our sensors also take advantage of this shift in orientation but do not require electrical input. Instead the change in orientation is driven by the presence of specific chemical species."

To create the sensors, the team engineered gold surfaces with nanoscale channels that hold a layer of receptor molecules such as carboxylic acids or metal carboxylates. The liquid crystal layer attaches to the receptors with weak interactions such as hydrogen bonds. Targeted chemicals displace these bonds freeing the liquid crystals to change their orientation and appearance and align in the channels.

The research team has demonstrated the device's sensitivity to parts-per-billion vapor concentrations of both amine and organophosphorus compounds. Abbott says the sensors also can differentiate their target chemicals from water vapor, alcohol, acetone and other chemicals that commonly foil other systems. The researchers are working to extend the technology to detect biological entities such as viruses, bacteria and proteins.